Purging Machine

I was telling a new guy in the warehouse that we always purge the machines after use because its not good for the machine to leave any sitting material. But later, I got into a discussion with one of my co-workers about whether or not that's true. I'd always been taught to purge the machine and never really questioned it but my buddy was telling me it's a waste of material and that leaving the PP in the machine won't hurt it. I'm happy to admit when I'm wrong so long as I get the right answer in the process. What are your thoughts?